In the field of printed circuit board manufacturing there are known processes for obtaining printed circuit boards by laminating a photosensitive element onto a copper board, performing pattern exposure against a resist, removing the cured sections with a developing solution, performing etching or plating to form a pattern, and further releasing off the cured sections from the substrate. In such processes, the resist material used for etching, plating and the like is most commonly a photosensitive element comprising a support and a photosensitive resin composition layer, alternatively with a protective film laminated thereon.
With the high densification of printed circuit boards in recent years, demand is increasing for ever higher resolution and adhesion of photosensitive elements. At the same time, photosensitive resin compositions with high sensitivity, low plating bath contamination and low sludge properties are desirable from the standpoint of improved workability.
It has been attempted to improve the properties of photosensitive resin compositions in order to meet such demands. Specifically there have been proposed, for example, photosensitive resin compositions with selective types and amounts of photopolymerization initiators (see German Patent Specification No. 2027467, European Patent Application No. 11786, European Patent Application No. 220, European Patent Application No. 589 and Japanese Patent Application Laid-Open No. 6-69631, for example), photosensitive resin compositions employing the photoinitiator 2,4,5-triphenylimidazole dimer and a hydrogen donor compound in combination (see U.S. Pat. No. 3,479,185, for example), photosensitive resin compositions comprising 2,4,5-triphenylimidazole dimer and a heterocyclic compound (see U.S. Pat. No. 3,647,467, for example), and photosensitive resin compositions with an oxazole compound added thereto (see Japanese Patent Application Laid-Open No. 62-180355, for example).
Problems to be Solved by the Invention
Upon detailed research on conventional photosensitive resin compositions including those described in above documents, the present inventors discovered that such conventional photosensitive resin compositions cannot be considered to satisfy all of the required demands such as sufficiently high photosensitivity, satisfactorily minimal plating bath contamination and satisfactorily minimal sludge generation during the developing step, as well as adequately high resolution and adhesion when used in a photosensitive element.
Specifically, in the photosensitive resin compositions described in German Patent Specification No. 2027467, European Patent Application No. 11786, European Patent Application No. 220, European Patent Application No. 589 and Japanese Patent Application Laid-Open No. 6-69631, for example, the effect of the selected photopolymerization initiator improves the photosensitivity but prevents satisfactory minimization of plating bath contamination, and therefore all of the demands mentioned above cannot be satisfied.
The photosensitive resin composition described in U.S. Pat. No. 3,479,185 allows plating bath contamination to be prevented to some degree, but it has been found that the combination of the 2,4,5-triphenylimidazole dimer and hydrogen donor compound does not provide a good balance between resolution and adhesion for photosensitive element use. That is, when a relatively large amount of 2,4,5-triphenylimidazole dimer is used, the resist line width is increased and resolution is reduced, while a relatively larger proportion of the hydrogen donor compound lowers adhesion with the copper board, and therefore it is clearly impossible to satisfy all of the demands mentioned above. Moreover, when the photosensitive resin composition contains a relatively large amount of added photopolymerization initiator, which has inferior solubility in organic solvents such as 2,4,5-triphenylimidazole dimer, sludge tends to be generated in the developing solution during the developing step.
It was also found that a photosensitive resin composition such as described in U.S. Pat. No. 3,647,467 fails to provide the photosensitive element with sufficient resolution and adhesion, and therefore cannot satisfy all of the aforementioned demands.
A photosensitive resin composition such as disclosed in Japanese Patent Application Laid-Open No. 62-180355 has also been shown to have relatively high adhesion for copper substrates when used in a photosensitive element, but its photosensitivity is not high enough and the aforementioned demands therefore cannot all be satisfied.
It is therefore an object of the present invention, which has been accomplished in light of the circumstances described above, to provide a photosensitive resin composition with sufficiently high photosensitivity, satisfactorily minimal plating bath contamination, and satisfactorily minimal generation of sludge during the developing step, as well as sufficiently high resolution and adhesion when used in a photosensitive element.